This invention relates generally to the field of machine control and, more particularly to analog adjustment of machine operation during the machine cycle.
The invention is particularly adapted for use in controlling machines where the operation is varied many times during each machine cycle. For example, in parison extrusion machines utilized in blow molding processes, the parison tooling orifice is frequently changed during the extrusion cycle to vary the parison wall thickness along the length of the extruded tube. The extruded parison tube is then placed in a mold and expanded by gas pressure introduced into the tube to fit the mold shape. By varying the wall thickness along the length of the extruded parison tube, the tube can be expanded to fit the inner surface of the mold to produce a finished product with any desired wall thickness profile.
In order to control the tooling orifice of an extruder to vary the parison tube wall thickness, controllers of the type described in the Hunkar patent, U.S. Pat. No. 3,712,772 have been developed. The controller there described includes a matrix-type patchboard in which the desired parison wall thickness is programmed for each time interval of the extrusion cycle. The patchboard is periodically scanned by an electronic timing mechanism to accordingly adjust the extruder tooling orifice. By changing the tooling orifice, the parison wall thickness is varied during the extrusion cycle to produce a parison tube with the desired wall thickness profile along the tube length.
In the operation of parison extrusion machines, it has been found that large step-like changes in the tooling orifice are undesirable. When a large step change occurs in the tooling orifice during extrusion of a parison tube, a ring is produced in the parison tube which, after the tube has been expanded into the mold, produces a noticeable and unattractive ring in the finished product.
For other types of machines in which the operation is varied at intervals in the machine cycle, large step changes in the control voltage may give rise to undesirable machine operation. For example, a large step change may cause undesirable overshoot of the controlled mechanism. As such, the finished product may have an undesired appearance or other undesired feature.
While controllers of the type described in the above mentioned patent have been well accepted, it has been found that the extrusion cycle must be divided into a large number of sequential intervals in order to generate the analog control signals for controlling the tooling orifice at a fast enough rate so that the largest step change in control voltage is small to thereby avoid the problem associated with large step changes during the extrusion cycle. For typical parison extrusion applications, it has been found that as many as 256 program points are necessary in order to prevent the problems of step changes. For controllers of the type described in U.S. Pat. No. 3,712,772, such a large number of program points cannot be accommodated without significant design changes being made which add to the cost of the controller. Even if a controller were available with 256 programmable points, operator inconvenience occurs because 256 changes have to be made in the controller patchboard to change the parison profile from one application to another.
An alternative approach to solving the problem has been to utilize an RC type circuit to generate a varying control voltage during each machine cycle interval. Because the RC circuit uses a resistor/capacitor network, the analog output thereof follows a curve with a fixed time constant. Because the control function generated by the controller usually has both large and small control voltage changes from one machine cycle interval to another, an RC circuit with a fixed time constant cannot smoothly vary the control voltage during a fixed time interval for all such voltage steps encountered in a typical application. For example, an RC interpolator designed to smoothly change the voltage during a given time period between two different amplitudes will produce a step-like voltage change during a time period of the same length where the voltage difference between the two amplitudes is significantly smaller than the first two amplitudes. As such, the RC circuit does not produce a smooth control voltage function nor does it solve the problem of large voltage steps.